Russian investigators probing the fatal Sukhoi Superjet 100 landing accident at Moscow Sheremetyevo are to analyse the reasons behind heavy-handed manual inputs by the captain, after the aircraft was hit by lightning and dropped into direct flight law.
Some 5s after the lightning strike the captain took manual control of the aircraft, which had been conducting a climbing right turn at the time.
The captain's initial inputs deflected the side-stick 11.7° to the left, more than half its travel range, and pushed it forward by 6.8°, equivalent to half the pitch-down travel range.
Further side-stick inputs – in both the roll and pitch axes – were "of an abrupt and intermittent character", says the Interstate Aviation Committee in a preliminary analysis of the 5 May accident.
It says the pilot turned the aircraft to the right using "multiple impulse deflections" of the side-stick, ranging from 30% to 65%, performing more than 10 roll-deflection movements over the course of just 18s in order to set a 20° bank.
The inquiry also says the 'priority' button on the left side-stick was also momentarily pressed six times.
Investigators compared the captain's side-stick inputs on the aircraft with those of a number of previous flights when the landing was conducted manually in normal flight law.
The inquiry says the comparison shows pitch movements for the ill-fated flight were characterised by "significantly wider amplitude" and "oscillatory" movements, which resulted in "significant changes of longitudinal motion parameters".
Similar "sweeping" movements, it states, were observed during direct-law landings performed by other Aeroflot crews.
The inquiry says the reasons for these particular piloting actions will be assessed during the preparation of the final report.
As a result of the lightning strike the crew chose to return to Sheremetyevo, turning south towards the airport.
During the initial attempt to position the aircraft for approach, the crew informed air traffic control that they were not ready and requested an orbit.
But the right-hand orbit, performed at 600m above the ground, proved difficult for the captain who "could not maintain the altitude precisely", says the inquiry. The aircraft rolled up to 40° during the right turns and the aircraft deviated from its height by up to 60m, triggering multiple aural alerts.
The crew prepared for an ILS approach to runway 24L but performed neither a before-landing briefing nor the approach checklist, and did not set the go-around altitude.
Investigators also note that there was "no discussion" by the crew of an 11s windshear warning as the aircraft descended through 1,100ft above ground, even though it would normally require the execution of a go-around unless the crew was certain of no windshear hazard.
As the aircraft reached the decision height at 270ft there was a notable rapid increase in its downward deviation from the glideslope, and the captain increased engine thrust – resulting in the aircraft's accelerating to 170kt by the time it was 16ft above the runway.
The captain then brought the thrust levers back to idle, in response to an automated 'retard' call, and initiated the flare by pulling back on the side-stick by 65% of its travel range.
But the captain's side-stick pitch inputs featured "ever-increase amplitude" – up to the maximum forward and aft positions, with long holds in both – resulted in oscillations in pitch as the aircraft touched down. It bounced several times, suffering substantial damage during the third impact and caught fire, eventually veering off the runway.